Sum of minimum number of vertices

Geometry Level 5

At t = 0 t = 0 , I begin truncating a cube. At t = 1 t = 1 , I obtain its dual, the octahedron.

Find

S 2 { 0 , 1 } min t ( 0 , 1 ) S V ( t ) \sum_{S \in 2^{\lbrace 0,1 \rbrace}} \min_{t \in (0,1) \cup S} V(t)

where V ( t ) V(t) is the number of vertices of the truncated cube at time t t and 2 A 2^{A} is the power set of A A .


The answer is 32.

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1 solution

Jake Lai
Apr 8, 2015

We are considering the sum over the set 2 { 0 , 1 } = { , { 0 } , { 1 } , { 0 , 1 } } 2^{\lbrace 0,1 \rbrace} = \lbrace \varnothing, \lbrace 0 \rbrace, \lbrace 1 \rbrace, \lbrace 0,1 \rbrace \rbrace . This just means we are summing the minima over the intervals ( 0 , 1 ) , [ 0 , 1 ) , ( 0 , 1 ] , (0,1), [0,1), (0,1], and [ 0 , 1 ] [0,1] .

In the interval ( 0 , 1 ) (0,1) , there is a point in time t r t_{r} where the cube is rectified, becoming a cuboctahedron. At this point, V ( t r ) = 12 V(t_{r}) = 12 , as compared to V ( t ) = 24 V(t) = 24 elsewhere. As such,

min t ( 0 , 1 ) V ( t ) = 12 \min_{t \in (0,1)} V(t) = 12

At t = 0 t=0 , we have a cube with V ( 0 ) = 8 V(0) = 8 . At t = 1 t=1 , we have an octahedron with V ( 1 ) = 6 V(1) = 6 . Thus,

min t [ 0 , 1 ) V ( t ) = 8 \min_{t \in [0,1)} V(t) = 8

min t ( 0 , 1 ] V ( t ) = 6 \min_{t \in (0,1]} V(t) = 6

min t [ 0 , 1 ] V ( t ) = 6 \min_{t \in [0,1]} V(t) = 6

Hence,

S 2 { 0 , 1 } min t ( 0 , 1 ) S V ( t ) = 12 + 8 + 6 + 6 = 32 \sum_{S \in 2^{\lbrace 0,1 \rbrace}} \min_{t \in (0,1) \cup S} V(t) = 12+8+6+6 = \boxed{32}

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